When Texas won its independence, it didn't just immediately become a part of the United States. For a while, it was an independent Republic with its own laws and own president. They made a deal to enter the United States but they didn't want to do it immediately because they didn't want their individual state rights to be infringed upon. Their culture and behavior was not so much general pro-american as it was Southern in its core and it's always wanted to be in charge of its own territory.
